1. Field of the Invention
The present invention relates generally to high energy chemical lasers and, more particularly to difficulties in fabricating a gain generator for a high energy, high pressure, deuterium fluoride (DF) chemical laser or hydrogen fluoride (HF) chemical laser.
2. Description of the Prior Art
A high energy DF or HF chemical laser system generates a laser beam in a gain generator assembly. Basically, the gain generator assembly contains a laser cavity and cavity injectors for the introduction of deuterium (D2) or (H2), atomic fluorine (F) and other gases. The gain generator also includes reactant manifolds and a high-pressure combustor that produces atomic fluorine required for lasing. A practical high energy DF or HF chemical laser system also includes a fluid supply assembly, a pressure recovery assembly, and a laser optics assembly, but these components are not pertinent to the present invention, which is concerned only with the fabrication and structure of the gain generator assembly.
A gain generator assembly typically includes a high pressure, high temperature combustor and a laser cavity and also includes a very large number of very small internal passages for introduction of fluids into the combustor and the cavity. One type of combustor and cavity injector structure used in prior gain generator assemblies was known as the high pressure hypersonic wedge (HPR-HYWND). This type has been chosen for use in a high energy laser demonstration systembecause of its relatively low cost and in spite of its relatively low efficiency. A more efficient gain generator technology was developed for the Mid-infrared Advanced Chemical Laser (MIRACL). Future high energy laser systems would benefit significantly by the use of MIRACL technology. However, fabrication of a gain generator assembly using the MIRACL technology required extensive chemical milling, diffusion bonding, electro-deposited nickel buildups, conventional machining, and the formation of literally millions of small holes of approximately 0.005 inch (13 xcexcm) diameter by electrical discharge machining (EDM). Unfortunately, these fabrication complexities are immensely multiplied when dealing with gain generators of high energy. The nickel deposition process deposits only 0.001 inch of nickel per hour. Multiple deposition cycles of many hours would be required, and cooling and feed passages must be machined into the deposited nickel and then closed off with additional deposited layers. Machining millions of small trip injection holes added millions of dollars to the cost. Another related difficulty is that larger versions of the gain generator assembly using MIRACL technology suffer from laser beam degradation resulting from wakes induced by internal manifolding and support struts needed to stabilize relatively long blade structures. Prior to the present invention, it was believed to be impractical to fabricate a gain generator assembly using the MIRACL technology, at at reasonable cost, and with a laser beam of acceptable quality.
MIRACL type gain generators utilized prior to the present invention were cooled only by the flow a helium/deuterium gas mixture that flowed through cavity injector blades and was injected into the laser cavity. Although this self-cooled configuration exhibited some desirable attributes, thermal and density gradients in the injected gases caused large unacceptable optical quality degradation in the laser cavity.
The gas cooled design required a tortuous gas cooling passage configuration that exhibited very high pressure losses. Consequently, very high inlet pressures, up to approximately 1,500 psia, were required. In addition, gas cooling necessitated relatively high operating temperatures and, therefore, the use of unalloyed nickel on surfaces exposed to high temperature fluorine gas. Nickel is weaker than high strength nickel alloys, and at high temperatures the nickel structures have high strain rates exceeding the elastic limit of the nickel material. This results in a low cyclic fatigue life of only approximately 1,000 cycles.
It will be appreciated from the foregoing that there is a significant need for improvement in the design of high energy chemical laser gain generator assemblies and in methods for their fabrication. In particular, what is needed is a high energy gain generator assembly that is capable of MIRACL type demonstrated efficiency, operation at relatively low inlet pressure, high or unlimited cyclic fatigue life, but which may be fabricated more conveniently and at less cost than gain generator assemblies of the prior art. The present invention satisfies these requirements.
The present invention resides in a high energy DF or HF chemical laser gain generator and a method for it""s fabrication. Briefly, and in general terms, the laser gain generator of the invention comprises a combustor for generating atomic fluorine (F), including a plurality of combustor injectors, for injecting into the combustor a gas containing fluorine and hydrocarbon fuel (or D2 in the case of an HF laser); a laser cavity in which lasing takes place as a result of a chemical reaction between the atomic fluorine (F) and deuterium (D2) or H2; and a plurality of laser cavity injector blades, for injecting deuterium (D2) or hydrogen (H2) with the atomic fluorine into the laser cavity. The laser cavity injector blades include internal passages for flow of cooling water or other cooling media. The gain generator is formed from a plurality of thin platelets of metal in which all required internal passages for the flow of cooling media and for the flow of diluent and reactive gases are formed by chemical etching of each platelet separately. Because the water-cooled or other cooling media laser gain generator operates at relatively low temperatures, it avoids the need for high gas inlet pressures. Further, lower operating temperatures permit the use of a high-strength alloy for the metal platelet material, thus avoiding the need for supporting structures within the gain generator. In the presently preferred embodiment of the invention, the alloy is Inconel 718 alloy, although other corrosion resistant super alloys such as Haynes alloy L605 could be utilized.
The method of the invention comprises the steps of separately etching each of a plurality of thin metal platelets, to define successive cross sections of a laser gain generator that includes a plurality of laser cavity injector blades with gas passages for the injection of fluorine and deuterium gases, and passages for the flow of cooling media such as water; stacking the etched metal platelets in alignment to form the laser gain generator; and applying heat and pressure to the stacked metal platelets, to fuse them together by diffusion bonding. More specifically, the step of separately etching includes forming cross-sectional slices of a plurality of cooling and gas passages within each of a plurality of cavity injector blades in the laser gain generator.
It will be appreciated from the foregoing that the present invention represents a significant advance in the field of high energy DF or HF chemical lasers. In particular, the invention provides for operation of a laser gain generator at relatively low metal temperatures and gas inlet pressures. The lower temperatures permit use of high-strength super alloy and this, in turn, avoids the need for supporting structures that would otherwise adversely affect laser beam quality. In addition the use of platelet fabrication technology lowers the manufacturing cost dramatically. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.